In the semiconductor manufacturing industry, photoresist materials are used for transferring an image to one or more underlying layers, such as metal, semiconductor and dielectric layers, disposed on a semiconductor substrate, as well as to the substrate itself. To increase the integration density of semiconductor devices and allow for the formation of structures having dimensions in the nanometer range, photoresists and photolithography processing tools having high-resolution capabilities have been and continue to be developed. To improve lithographic performance, 193 nm immersion lithography tools have been developed to effectively increase the numerical aperture (NA) of the lens of the imaging device, for example, a scanner having an ArF light source. This is accomplished by use of a relatively high refractive index fluid (i.e., an immersion fluid) between the last surface of the imaging device and the upper surface of the semiconductor wafer. The resolution limit of 193 nm immersion scanners, however, does not allow for direct patterning of certain features at the resolutions needed in next generation semiconductor devices. To increase imaging resolution over that obtainable with 193 immersion scanners, EUV exposure tools have been developed. The widespread adoption of these tools has, however, been delayed due, for example, to technical issues surrounding the EUV light source and their prohibitively high cost. Accordingly, it would be desirable to extend the capabilities of existing lithographic toolsets through new process integration schemes.
A double patterning process has been proposed for the printing of high density contact hole arrays. In this process, separate exposure steps using different photomasks are used to image a first contact hole pattern and then a second contact hole pattern offset from the first pattern. This method, however, is highly sensitive to overlay error in that precise alignment of the second contact hole mask pattern for the second exposure to the first exposure contact hole pattern is required.
To avoid the potential for alignment error, a self-aligned contact hole formation method has been proposed. With reference to FIG. 1, U.S. Pat. No. 8,309,463 B2 discloses formation of a spacer layer 1 on sidewalls of photoresist pillar patterns 2 disposed in a square array such that adjacent spacers come into contact with each other, removing the photoresist patterns, etching an underlying mask layer 3 using the spacer as an etch mask to form a mask pattern and etching an underlying target etching layer using the mask pattern as an etch mask to form a target contact hole. Contact holes are formed in the regions 4 between (external to) the spacers and in the regions within the spacers. The spacer layer can be a nitride film, an oxide film or a combination thereof, and can be formed by atomic layer deposition (ALD). This process is disadvantageous in that the regions formed between the spacers are not round, but rather have an astroid shape in which a cusp 5 is formed at the contact point of each set of adjacent spacers. It is believed that this pattern would be transferred to the mask layer and target contact hole after etching. As a result of the pattern's non-circular geometry and cusps, it is believed that electrical characteristics of the resulting devices would be detrimentally affected. In addition, differences in size and geometry of contact holes formed from the regions between the spacers with respect to the surrounding holes formed from the regions within the spacers can result in a bimodal distribution of conductivity, which can also adversely impact device performance. The described process is also disadvantageous in that non-square contact hole arrays can exacerbate the problems described above. For example, in the case of an oblong rectangular contact hole array, the resulting contact holes formed between pillar patterns would themselves be oblong as well as having cusps, making those holes differ in size and geometry from the circular holes formed from the regions within the spacers. A hexagonal contact hole array would result in a deltoid-shaped region with three cusps between each group of three surrounding pillars, and would suffer from the same problems described above with respect to the square array.
There is a continuing need in the art for improved pattern forming methods which address one or more problems associated with the state of the art and which allow for the formation of high-density contact hole patterns in electronic device fabrication.